Top slot sizing fix and sample update

We were drawing dimensions for the top slot from resize events, but these ended up forcing a scrolling top slot to the size of the containing window, not its contents, which was wrong.

Responsibility for this is now moved to the `ShoesLayout` itself, which does a better job of it.

Fixes #617.

With the background issue resolved, there's no reason for samples/expert-funnies.rb to be excluded.

Except it doesn't work :( XKCD appears to have discontinued the RSS element that we were drawing our image from, and the code didn't handle it gracefully.

So now we look to see if we get that parsed out right before adding a section, so things are a bit more resilient.